Story related, can't be missed
After arriving at the island, go through the cave to the left and look at the red boards on the rock out the window. Double back and continue on the path to the left. Once you reach the door, hit the left lever and turn around to see something written on the back of a board. Follow it to a box with a clue. Double back to the nearest platform on the left to find the missing piece. Return it to the door, throw the switch and proceed through.
Explore this area to find a bridge requiring a five symbol code. Just putting in the code here does NOT work, you must fill in your journal first. Scour the area for the remaining four symbols on red boards and note their missing prongs at the top. You must interact with each of them to fill in the journal and the code be accepted. They can be found in the following places:
Sun- on the lower path past the campsite
Bird- parakeet area at the end of the paths
Mountain- on the upper path past the bridge
Water- already found in the cave at the beach
Fishing- bottom of both paths by the hut
If you get stuck...